With the increasing use of wireless networking, especially 802.11-based wireless systems in enterprise networks, there is now a desire to develop services that provide more than untethered network access. An important class of services includes those services that use end-user location information. Such services include location-aware content delivery, emergency location, services based on the notion of closest resource, and location-based access control. Techniques that can estimate location in indoor environments are important to enable such services in enterprises.
In typical wireless deployments in an enterprise, a site is served by several access points (APs). Client devices associate with an AP to obtain connectivity. Indoor wireless LAN (WLAN) location estimation can employ one of several physical attributes of the medium for estimation. The typical features that might be used are: the received signal strength (RSS) of communication, the angle of arrival of the signal, and the time difference of arrival. Among these, RSS is the only feature that is measurable with reasonably priced current commercial hardware. Related efforts have used RSS for location estimation and concentrated on locating a user in two-dimensional space, e.g., a point on one floor of a site.
Conventional techniques for WLAN location estimation operate in two phases: a model building offline phase, and an online phase where estimation is performed. The model constructed in the offline phase is, essentially, a map of signal strength behavior at the site. The model is either constructed using many measurements at the site, or is parametric and depends on several variables like type and number of walls and other signal obstructers. In the online phase, a set of signal measurements is mapped to a location after “consulting” the model. The accuracy of estimation depends on the technique used to build the model, and the methodology used to match the measured signal strengths to the model.
The deployment of location estimation systems is typically done in either a client-based deployment or an infrastructure-based deployment. In the client-based deployment, the client, in the online phase, measures the signal strengths received from various APs. This information is used to locate the client. In the infrastructure-based deployment, the administrator deploys sniffing devices that monitor clients and the signal strength from clients. This sniffed information is used to locate the clients in the online phase. For ease of management, including provisioning, security, deployment and maintenance, and the fact that no client changes are required, most enterprises tend to prefer the infrastructure-based deployment.